Isotopic evidence for axial tree stem methane oxidation within subtropical lowland forests

Knowledge regarding processes, pathways and mechanisms that may moderate methane (CH₄) sink/source behaviour along the sediment - tree stem - atmosphere continuum remains incomplete. Here, we applied stable isotope analysis (δ¹³C-CH₄) to gain insights into axial CH₄ transport and oxidation in two common and globally distributed subtropical lowland forest species (Melaleuca quinquenervia and Casuarina glauca). We found consistent trends in CH₄ flux (decreasing with height) and δ¹³C-CH₄ enrichment (increasing with height) in relation to stem height from the ground. The average lower tree stem (0-40 cm) δ¹³C-CH₄ of M. quinquenervia and C. glauca flooded forests (-53.96 ‰ and -65.89 ‰) were similar to adjacent flooded sediment CH₄ ebullition (-52.87 ‰ and -62.98 ‰), suggesting that CH₄ is produced mainly via sedimentary sources. Upper stems (81-200 cm) displayed distinct δ¹³C-CH₄ enrichment (M. quinquenervia -44.6 ‰ and C. glauca -46.5 ‰ respectively) compared to lower stems. Coupled 3D photogrammetry and novel 3D measurements on M. quinquenervia revealed that distinct hotspots of CH₄ flux and isotopic fractionation were likely due to bark anomalies where preferential pathways of gas efflux were likely enhanced. By applying a  fractionation factor (derived from previous lab based tree stem bark experiments), diel experiments revealed greater δ¹³C-CH₄ enrichment and higher oxidation rates in the afternoon relative to the morning. Overall, we estimate CH₄ oxidation rates between the lower to upper stems across both species ranged from 1 to 69 % (average 33.1 ± 3.4 %), representing a substantial tree-associated CH₄ sink occurring during axial transport.